1. Field of the Invention
The present invention relates to a method of producing a silicon single crystal by the Czochralski method (hereinafter referred to as the CZ method).
2. Description of Related Art
It is a general practice in producing a silicon single crystal by the CZ method that raw materials put in a crucible, for example, inside a chamber are heated and melted by a heater, and then a seed crystal is dipped in this molten liquid and pulled upwards while it is being rotated, whereby a silicon single crystal grows at the lower end of the seed crystal.
In the case where a semiconductor integrated circuit is manufactured by the use of a silicon single crystal substrate, in order to give an appropriate electric conductivity to the substrate, it has been required to contain an adequate amount of dopants, such as antimony (Sb), boron (B) and phosphorous (P). In addition, an adequate amount of oxygen should be contained in the substrate of silicon single crystal so as to gain a so-called Intrinsic Gettering (IG) effect. "Gettering" means a process that removes harmful impurities, e.g. heavy metals, from the regions in a wafer where devices are fabricated. "Wafer" means disk-shaped substrate. IG uses the defects associated with oxygen precipitation in the interior of the wafer, for trapping sites for the harmful impurities. (S. M. Sze, "VLSI Technology" .sctn.1.5.1. McGraw Hill, 1983). Therefore, if the silicon single crystal is employed for a substrate, oxygen of a proper and uniform concentration is necessary. For this purpose, the oxygen concentration in the molten liquid of raw materials in the crucible should be controlled.
The oxygen which is supplied into the molten liquid from the surface of the quartz crucible through contact therebetween is stirred by a forced convection of the molten liquid due to the rotation of the crucible and a heat convection of the molten liquid due to the temperature difference thereof in the crucible. During stirring, the oxygen is not only evaporated from the surface of the liquid in the form of silicon monoxide (SiO), but is carried to the growth surface of the silicon single crystal and taken thereinto.
As one method to control the oxygen concentration taken in the silicon single crystal, Japanese Patent Laid-open Publication Nos. 57-27996 (27996/1982) and 57-135796 (135796/1982) disclose, with noting the relation between the rotation rate of the crucible and the oxygen concentration in the silicon single crystal, changing the rotation rate of the crucible in relation to the amount of the molten silicon liquid, thereby changing the relative speed of the quartz crucible and molten silicon liquid, which results in a forced convection of the molten silicon liquid. Thus, it becomes possible to adjust the thickness of a boundary layer where the oxygen is diffused in the surface of the quartz crucible by the forced convection of the molten liquid. Japanese Patent Laid-open Publication No. 62-153191 (153191/1987) discloses another method, wherein the contact area between the quartz crucible and molten silicon liquid and the temperature of the molten silicon liquid are changed. Specifically, according to this latter method, while the supplying ratio of electric power to a plurality of heaters provided in the periphery of side walls of the crucible is adjusted, and accordingly a part of the raw materials are kept in the solid state in the crucible, the silicon single crystal is grown.
Recently, in order to produce a silicon single crystal substrate capable of being precisely processed with an increase of the integration degree of semiconductor integrated circuit, Sb has been used as the dopant to be added to the silicon single crystal substrate in many cases. However, a problem has occurred in that the oxygen concentration in the silicon single crystal doped with Sb is lower than that in the silicon single crystal without dopants or the silicon single crystal with other dopants, for example B and P, so that it is difficult to make the oxygen concentration in the silicon single crystal high (15.times.10.sup.17 atoms/cm.sup.3 or more) sufficient for obtaining the IG effect by the above-described method of controlling the oxygen concentration.
This is based on a property that a vapor pressure of diantimony trioxide (Sb.sub.2 O.sub.3) formed by the addition of Sb is higher than that of SiO, so that Sb.sub.2 O.sub.3 is easily evaporated from the surface of the molten silicon liquid. It has not been possible to solve this problem by the above-described method of controlling the oxygen concentration.